Technical Field
The present disclosure relates generally to the field of combustion furnaces and methods of use, and more specifically to submerged combustion melters and methods of use in producing molten glass and similar materials using one or more measured temperatures for control of a submerged combustion melter, and systems for carrying out such methods.
Background Art
In submerged combustion melting of glass and similar materials, combustion gases emitted from sidewall-mounted and/or floor-mounted burners are injected beneath the surface of the molten mass and rise upward through the melt. The material is heated at a high efficiency via the intimate contact with the combustion gases. Using submerged combustion burners produces violent turbulence of the molten material, which may include unmelted material as well as gases. Vibration of the burners and/or the melter walls themselves, due to sloshing of molten material, pulsing of combustion burners, popping of large bubbles above submerged burners, ejection of molten material from the melt against the walls and ceiling of melter, and the like, are possible. Frequently, one or more of these phenomena may result in undesirably short life of temperature sensors and other components used to monitor a submerged combustion melter's operation, making monitoring difficult, and use of signals from these sensors for melter control all but impossible for more than a limited time period. Given that long melter life, and control of the melter during that life are goals for submerged combustion melters and sensors, this failure of sensors is a significant detriment to those goals. Submerged combustion has been proposed in several patents for application in commercial glass melting, including U.S. Pat. Nos. 4,539,034; 3,170,781; 3,237,929; 3,260,587; 3,606,825; 3,627,504; 3,738,792; 3,764,287; 6,460,376; 6,739,152; 6,857,999; 6,883,349; 7,273,583; 7,428,827; 7,448,231; and 7,565,819; and published U.S. Patent Publication numbers 2004/0168474; 2004/0224833; 2007/0212546; 2006/0000239; 2002/0162358; 2009/0042709; 2008/0256981; 2008/0276652; 2007/0122332; 2004/0168474; 2004/0224833; 2007/0212546; and 2011/0308280.
Rue, “Energy-Efficient Glass Melting—The Next Generation Melter”, Gas Technology Institute, Project No. 20621 Final Report (2008) noted that, in submerged combustion melters using oxy-fuel burners to melt glass precursors, one of the most difficult measurements to make is the actual melt temperature. In one try in a small melter, platinum-clad thermocouples failed when exposed directly to the melt after a short time period because of the interaction with oxy-fuel flame. Stable, controlled combustion of the fuel within the melt is required or highly desirable in submerged combustion melting, according to this report. There are three ways to attack this problem according to this report: flame stabilization, such as in U.S. Pat. No. 7,273,583; splitting the fuel-oxidant mixture into smaller jets; and/or preheating the fuel/oxidant mixture. However, there is no teaching or suggestion that accurate melt temperature may be indirectly measured and used to control operation of the melter to achieve a desired actual melt temperature. Rue also notes that the heat flux through the frozen or highly viscous melt layer (present in submerged combustion melters having fluid-cooled panels) is determined by the properties of the processed material and the temperature and turbulence of the melt. It is therefore undesirable to superheat the melt because this increases the heat flux through the walls (and frozen melt layer). Heat flux through cooled panel walls is relatively independent of the temperature of the coolant according to this report, since the thickness of the frozen layer compensates for any increase or decrease in coolant temperature. Therefore, knowledge of melt temperature is critical to controlling heat flux.
Muijsenberg, et al., “An Advanced Control System to Increase Glass Quality and Glass Production Yields Based on GS ESLLI Technology” 66th Conference on Glass Problems: Ceramic Engineering and Science Processings, Volume 27, Issue 1, Chapter 3, published online 26 Mar. 2008, noted that, in the context of conventional (non-submerged combustion) glass furnaces, when the glass production needs to produce products of consistent excellent glass quality at high yield and low energy usage, it is almost impossible to control the production manually. Therefore a group of advanced control techniques was developed for an automatic control. One commonly used is Model (based) Predictive Control (MPC). Correct usage of MPC together with knowledge of glass production, according to this article, results in process stabilization, increasing glass quality and energy savings.
An advanced temperature measurement system was developed for conventional glass furnaces including “self-verifying temperature sensors”, such as disclosed in U.S. Pat. Nos. 5,713,668 and 5,887,978. Even with these improvements, however, it is not clear if these temperature sensors would stand the rigors of highly turbulent submerged combustion melters. As noted above by Rue, the interaction with oxy-fuel flames would no doubt be severely detrimental to these sensors as well.
It would be a significant advance in the glass melting art to develop processes of operating submerged combustion melters, and systems to carry out the processes in producing molten glass and similar materials using one or more methods of indirectly measuring temperature of the molten material in the melting zone of the melter.